50% efficient cell to be CPV game changer

Traditional PV may boast rock-bottom costs, but could CPV’s potential for increased efficiency make it a better long-term investment?

If you want a leisurely chat with one of Solar Junction’s research and development (R&D) masterminds then now might be a good time to call.

The team that last April set a world record for commercial production solar cell efficiency, at 43.5%, has got a lot of time on its hands right now, says Solar Junction’s director of business development, Jeff Allen.

“Our R&D guys are going, ‘I want more tool time,’ and I am going: ‘no’,” he says. “We are telling them: ‘You can't have it.’ In fact you get less R&D time because we are making products and selling them to customers, and they can't get enough. We are booked for the entirety of 2012.”

Solar Junction has been able to fill its order books with, Allen says, “minimal effort” at a time when many solar businesses are wondering if they will make it through the next few years. That is testimony to the importance of cell efficiency in the economics of CPV.

And its importance is set to grow further, says Brett Prior, senior analyst at GTM Research. “We see cell efficiency going from 43% now up to 50% in the next decade. That drives down the cost. You just swap out the cells in your modules and that is a 20% improvement.

“It lowers your cost by 20%”

Such refinements cannot be achieved using single-junction cells. Tim Keating, vice president of marketing and field operations at Skyline Solar, says: “The upper limit for any single junction is about 30% to 33%.”

For this reason, Solar Junction has focused on commercialising a multi-junction solar cell architecture where each junction (or ‘j’) can be tuned to a different wavelength of light.

Three-junction design

Allen says: “We can immediately translate our three-junction design, and with some effort, but not massive effort, we can go to four-j and five-j and even, frankly, six-j. For us it is a pretty trivial extension.”

He adds: “the terrestrial CPV companies are very excited with us.” And it is not difficult to see why. What he sees as a trivial extension is something that translates directly to a drop in cost, which nobody needs reminding is the most important factor for success in solar PV.

Today’s 43.5% top cell efficiency equates to a DC module efficiency of around 28.7% and a post-inverter AC system efficiency of around 24.6%, according to figures from GTM Research provided for the CPV Consortium last October.

GTM Research expects cell efficiency to rise to 50.4% by 2020, which tallies with Allen’s assessment of CPV’s future potential. “We have a roadmap,” he states. “If we press it aggressively it could take us to in excess of 50% cell efficiency within the decade.

“It is just a matter of time and money.”

If this happens, GTM Research figures suggest module efficiency could reach 40.5% and systems could become 35.8% efficient. Naturally, though, it will take more than just more efficient cells for CPV to outgun traditional PV.

In fact, says Keating: “Efficiency is kind of a red herring. What is important is the cost of electricity you produce.

Cell efficiency: what really counts

“If you get a 30%-efficient system that costs twice as much per kilowatt-hour as a competitor’s, you are not going to buy it. There is a sweet spot in the efficiency and cost-efficiency curve.”

There is no doubt a lot of truth in that, but it is also the case that cell efficiency currently represents one of the biggest opportunities for cost reduction in CPV.

By 2020, GTM Research expects CPV installed system costs to fall 61%, from USD$3.08 per watt currently to $1.20 per watt. Of that drop, $0.48 could be down to higher cell efficiencies, a figure only surpassed by potential reductions in system losses ($0.49 per watt).

These developments are expected to help drive down the levelised cost of energy for CPV, overtaking traditional PV any time now and heading to $0.06 per kilowatt-hour by 2020, compared to around $0.08 per kilowatt-hour other solar technologies.

Says Allen: “This is a key difference between flat-plate PV and CPV. In the case of flat plate, there is not really a lot of room to go in terms of increased efficiency. So all they can do is attempt to drive down costs. But even that is very difficult right now.

“You look at First Solar: they are having a struggle getting further down the cost curve. Compound that with the fact that some of these mega-companies need to get capacity. And going from 2GW to 4GW is a tall order. It takes a lot of money.”

He adds: “Now you contrast that to CPV. You have efficiency as a big lever and it is a less mature market in terms of shaking out the costs. So there is a lot of opportunity there.”